Just been comparing C.W.Dymond's 1877 ground plan to that of John Barnatts 1982 version. It is interesting that CW not only shows the ground plan layout but the 'lean' on the stones as well, indicated by the dotted lines. You will note on the far south-west stone on the left he shows said dotted lines as being away from the adjacent stone as well, where today they are touching. Of course the problem with this is that it doesn't show the 'middle' between the top and bottom where is actually touches so hopefully, if John Barnatt replies to my email he may be able to clear that up...certainly in his drawing some 105 years later anyway. https://picasaweb.google.com/100525707086862773355/DropBox?authkey=Gv1sRgCPmU2MWil9rM9AE#5852155605660104418

Roy, do you have a note of the heights for the westernmost side stones ?

George, the two most westerly support flankers are 8'-3" on the northern side and 7'-0" on the southern taken from the internal base level. There is a 15" difference in their height obviously. The prostrate stone is 8'-8" long/tall.
I've had to take thse measurements from the scale model as I can't find my original notes taken when I met the EH guy there last year at the moment! I remember them anyway so they are correct. The catflap stone is 10'-3" tall off the same base level.
As you will know the other flankers fall short of the Capstone.

PS. The northernly flanker measurement is taken from its highest point and the other from its central point so the overall difference is not 15" of course!

Thanks Roy .

If the prostrate stone was the backstone ,when erect it could have taken pressure off stone 4 and possibly stone 3 and possibly produced a relatively common portal dolmen appearance where the side stones are not there for structural support . ?

If the prostrate stone was a backstone George it wouldn't have touched anything other than the main eastern orthostat which it rested on. It would have been a trilithon with added flankers. The front flankers would have rested against the main orthostat as they do now and underlap the rear flankers which would have required banking to their base (as now) to stop them falling outwards. A pretty weak structure I would have thought considering the flankers are not earthfast. As it is, at least the rear flankers are secured in place both top and bottom by the banking and the pressure of the capstone pushing down on them and the overlapping stopping the other two from falling either way! Much more effective.

Why wouldn't the prostrate stone if erect , not touch and support the capstone ?
It looks as if there was a much bigger surrounding cairn that at present providing more stability for everything including the presently prostrate stone .

Sorry George I didn't word that correctly. The prostrate backstone (if it was a backstone that is) if erect, would support the capstone to the western end with the main eastern orthostate supporting the eastern end and have a slope of around 19". The capstone wouldn't touch any of the flankers and be a weaker overall structure as explaned in my previous post. That is why I am so concerned about the mounded base being worn away and not built back up. The recumbent stone is and has always been a talking point as to where it originated from with the backstone idea always being favourite...then again :-)

That has always been my point , if the prostrate stone is tall enough for a supporting role and we don't know the exact height it achieved , we have what is typical portal dolmen architecture , with the side stones not necessarily being involved in support although one may have been .
Not so different from local Zennor but having an extra side stone .

That's certainly one point of view and a valid one if the prostrate stone was a backstone. But if it wasn't and if one could show good reason why it wasn't then......

Not only valid but also the most parsimonious .

I believe it is also the most parsimonious

It's certainly more formal and 'easy' to build but did they always want stingy? Much classier if done properly and more impressive.
If it was the backstone, any thoughts on what caused it to fall George considering the huge weight holding it in place?

It's perfectly normal and typical for the type of monument to have a backstone .If that isn't the backstone where did the backstone get to and what is the stone doing there ? I don't see how , if that stone is the backstone the monument is any less impressive . Classier ,proper and stingy are merely subjective .
What they did was to build conspicuous constructions that showed off their skills . If you have to have a capstone you don't need to have them at the angle they are often found at ,and it needn't be be monstrous as in some cases even locally http://www.themodernantiquarian.com/site/643/pawton_quoit.html or further afield http://www.themodernantiquarian.com/site/2141/hendre_waelod.htmlhttp://www.themodernantiquarian.com/site/887/brownes_hill.html
Not all backstones at portal tombs are extant ,only 129 survive . Portal tombs collapse and are pushed over , the cairn is almost non existent at the rear which might be telling .

Well, we'll soon see...there is a backstone there, but I don't believe it is the prostrate stone! That's a brave claim isn't it...or is it if you are observant and have the opportunity to visit and study a site every day if you choose to? I admire you for your apparent knowledge George but not everything always has to be like what has gone before, far from it. My findings will surprise you and a lot of people I promise now that my research is complete. At the moment Trethevy Quoit doesn't 'look right'. I make it right using no more than is already there.

I'm not sure if brave is appropriate Roy . From what you seem to be suggesting you are applying what you think should be the architecture based on what you would do if you were the builder and a view that sees the monument as not "looking right " . If you don't accept the obvious backstone as being the backstone and also accept that there was a backstone then there is a much more complex series of events needed to support that belief with the consequent problem of finding eveidence to support that . I hope that the minimal amount of knowledge needed to comment was more substantial than just apparent knowledge .

The complex series of events are all there and all covered with real evidence George not hearsay or a vivid imagination. Just wait and see. You might even say 'well done' afterwards :-)

Ok , it all hinges on the evidence . If I'm convinced I'll be the first to say more than "well done " .